Sintered moldings, whether made out of metallic or plastic components, and methods of making sintered molding, for instance, common compression molding or plastic and metal injection molding (MIM), are used in many applications in the industry since, on the one hand, even complex moldings with a high accuracy of shape and high stability can be manufactured in large-scale production and, on the other hand, a lot of energy is saved with the application of sintering processes. At first, a preformed molding, also called green compact, is generally manufactured on the basis of a sinterable powder mixture, which is converted to a stable and true-to-form molding in a sintering phase. Pressing aids are typically used in the sinterable powder mixture. Such aids often are based on compounds such as metal stearates, amide waxes, including fatty acid amides or the like, which improve the conductivity and compressibility, and provide the green compact with a certain molding stability, particularly green strength, for the further processing prior to and during the sintering step. As a general rule, the pressing aid is removed prior to the sintering step or is burned out of the part during the sintering step.
The preformed green compact obtains an increased stability and rigidity during the sintering process as a result of binding of the individual powder particles to each other. Depending on the composition of the mixture used, the sintering conditions, and density of the components, pressing aids with very good lubricating effects can result in creation of carbon (“soot”) with resultant stains on the surface of the sintered part. These are presumably generated by pyrolysis of the pressing aids used on the component surface, either during the actual sintering step or in a preceding heating step with temperatures clearly above the melting point of the pressing aid, both of which steps are intended to remove the pressing aid(“debinding”).
US 2004/0138049 A1 reveals an improved composition consisting of an agitating agent, which would be suitable for metallic powder injection pressing. The agitating agent is a gaseous agent, which makes internal agitation available, which facilitates solvent-based binder extraction. Preferred agitating agents are selected from a group consisting of metallic bicarbonates, carbonates from metals of group I of the chemical periodic system, metal carbonates, metal bicarbonates and combinations of the same.
US 2003/0220424 A1 reveals a binder composition consisting of an aliphatic polyester-polymer, an ethylene bisamide wax and a guanidine moist agent. This mixture can also consist of an accelerant for the debinding process, which is preferably selected from organic or inorganic peroxides, azo-compounds or metallic compounds.
Hence, the present invention provides a mixture, particularly for sinterable powder mixtures, which produces less surface staining.